The Drosophila compound eye consists of 800 ommatidia, each of which contains two visual systems that perform different functions. The specific aims of the proposed experiments are: (1) Investigate the direction and timing of the signal between R7 and R8 that determines which pair of rh is expressed in a given ommatidium (p vs. y); (2) Test the model that the Rhodopsins themselves are able to influence the fate of a photoreceptor and its brain projection, and are also involved in the communication between R7 and R8; (3) characterize and identify genes specific to each subtype of photoreceptors, using p- and y-specific cDNA libraries and differential display; (4) Design a dominant genetic screen to identify factors responsible for the precise p/y ratio, using a technique to identify p or y ommatidia in live flies with GFP; and pursue a recessive mosaic screen to identify functions necessary for proper rh expression and for the communication between R7 and R8. This work may lead to an understanding of how communication among sensory receptor cells, as well as between these cells and the brain structures, allows establishment of the correct connections during development for processing sensory information.